1. Field of the Invention
The present invention relates to a multi-layer display element having a plurality of display panels placed in layers, and a method of fabricating the same.
2. Description of the Related Art
In the future, it is expected that electronic paper rapidly spreads, which can hold display images with no power supply and electrically rewrite a display content. Electronic paper is under development with the aim of realizing ultralow power consumption enabling memory display even though a power supply is turned off, reflective display easy on the eyes with no fatigue, and a flexible, low-profile display body with flexibility like paper. As the applications of electronic paper, electronic books, electronic newspapers, and electronic poster are proposed.
Depending on the differences of display types, electronic paper is categorized into an electrophoretic type, a twisting ball type, an electrochemical type, and a liquid crystal type. The electrophoretic type is a type in which charged particles are migrated in the air or in a liquid. The twisting ball type is a type in which charged particles tinted into two colors are rotated. The electrochemical type is a type using a display element having a structure in which a display material is sandwiched between electrodes, the display material being colored, faded or changed in color by an electrochemical reaction when an electric field is applied. A liquid crystal display element is a non-self-luminous display element having a structure in which a liquid crystal layer is sandwiched between a pixel electrode and a counter electrode.
The liquid crystal type is predominantly advantageous in color representation for electronic paper, which uses a liquid crystal display element (a liquid crystal display). In the liquid crystal type, a cholesteric liquid crystal type is advantageous, which uses a liquid crystal composition forming a cholesteric phase. This liquid crystal composition is referred to as cholesteric liquid crystals or chiral nematic liquid crystals. Hereinafter, it is referred to as “cholesteric liquid crystals”. In the cholesteric liquid crystal type, the voltage to be applied to liquid crystals is controlled to switch the states of liquid crystals between the interference reflection state (planar state) in which light at a predetermined wavelength is selectively reflected in a liquid crystal layer according to the helical pitch of liquid crystal molecules and the transmissive state (focal conic state) in which the liquid crystal layer loses the selectivity of the reflection wavelength to transmit most of the incident light. Because these two states are stably held for a long time after the voltage is removed, the cholesteric liquid crystal type can maintain images with no power supply. In addition, as to the colorization of displayed images, in the cholesteric liquid crystal type, liquid crystal display panels in three colors can be used as the panels are laid on one another. Thus, in the cholesteric liquid crystal type, a black light absorbing layer is provided on the rear side, whereby when all the liquid crystal display panels are turned into the transmissive state, the screen can be displayed black, whereas when a liquid crystal display panel in necessary color is turned into the interference reflection state, the screen can be displayed in color.
In addition, also in the electrochemical type, because the states can be switched between the transparent state of transmitting the incident light and the reflective or absorbing state, display panels are laid on one another for use as similar to the cholesteric liquid crystal type, whereby color representation is allowed.
On the other hand, except the liquid crystal type and the electrochemical type, the other types do not allow the light transmissive state. Therefore, when color representation is conducted in the other types, it is necessary that a color filter is arranged on the display surface, the color filter being applied with three colors, red (R), green (G) and blue (B), for example, in blocks, and each pixel is divided into three sub-pixels. On this account, in these other types, the lightness is one-third as compared with that of the cholesteric liquid crystal type. Therefore, among reflective display elements with no provision of a backlight unit or front light unit, multi-layer display elements using the cholesteric liquid crystal type or the electrochemical type in particular can obtain bright display, and these elements are predominantly advantageous in regard to color representation.
In the multi-layer display element having a plurality of display panels placed in layers, in placing display panels in individual colors in layers, it is necessary to align the positions of the individual display panels in the in-plane direction and bond the panels together so as not to misalign the alignment positions of the pixels of the display panels. When the positions of the display panels in the in-plane direction are displaced, the alignment positions of the pixels are misaligned to cause color shift or the degradation of brightness, which leads to a reduced sharpness of the edges of text and images. On this account, it is difficult to obtain excellent display quality.
In order to align (match) the alignment positions of display panels with each other, generally, alignment marks are formed on each of display panels, and an operator enhances and visually recognizes the marks with a microscope, for example. At this time, the positions of the display panels are aligned with each other such that the positions of the alignment marks have a predetermined relation between the display panels. However, when alignment marks in the same shape are formed on each of the display panels, it is difficult to distinguish between a plurality of the alignment marks laid on each other which alignment mark corresponds to which one of upper and lower display panels. On this account, the operator slightly moves any one of the display panels and determines that the moved alignment mark corresponds to the display panel moved by the operator. Therefore, extra work is required to suffer working efficiency. Moreover, also in the case in which alignment marks are imaged by a CCD (Charge-Coupled Device) camera to subject the taken image to image processing for detecting the positions of the alignment marks, it is difficult to distinguish between the alignment marks laid on each other, and it is not easy to match alignment positions with each other.
In order to solve the problem that it is difficult to distinguish between alignment marks laid on each other, it is possible to efficiently match alignment positions with each other when alignment marks are formed in different shapes for each of display panels.
In addition, in the multi-layer display element, because each of the display panels has a structure in which electrode substrates are paired and attached to each other, each display panel has some thickness. On this account, when the thickness of two display panels placed in layers is increased, it is difficult to visually recognize alignment marks at the same time, which leads to a degraded position accuracy of alignment positions, or to longer work hours required for alignment work.
Patent Document 1 (JP-A-2003-295148) discloses a method of improving the visibility of alignment marks in order to solve the problem that it is difficult to visually recognize the alignment marks of two display panels placed in layers at the same time. FIG. 17 is a plan view schematically depicting a liquid crystal display element 400 as disclosed in Patent Document 1. As shown in FIG. 17, in the liquid crystal display element 400, a dummy terminal forming area 418 is provided on each of transparent substrates 401 of a plurality of liquid crystal display panels 413, the dummy terminal forming area 418 is formed at the position corresponding to a terminal forming area of the adjacent liquid crystal display panel 413, and an alignment mark 419 is formed in the terminal forming area and the dummy terminal forming area 418 for use in placing the panels in layers.
In addition, not shown in the drawing, Patent Document 2 (JP-A-2001-343916) discloses a method of forming alignment marks in concentric circular shapes. In both of the methods, because the alignment marks are formed of a transparent electrode material, the contrast becomes low to cause insufficient visibility.
In order to solve the problem of insufficient visibility, Patent Document 3 (Japanese Patent No. 2548560) discloses a method in which alignment electrodes 518 and 519 are also formed outside a display area E of display panels 513, voltage is applied to the electrodes 518 and 519 formed outside the display area E to produce or erase color for providing contrast to the areas surrounding the electrodes when the display panels 513 are placed in layers, and thus alignment marks are provided. FIG. 18 is a plan view schematically depicting a previous liquid crystal display element 500 disclosed in Patent Document 3. As shown in FIG. 18, in the liquid crystal display element 500, one or more of terminals of the alignment electrodes 518 are formed as adjacent to a terminal group of display electrodes, the terminals of the alignment electrodes 518 having a width narrower than that of the terminal group of the display electrodes, and the alignment electrodes 518 are extended into a seal to indicate alignment marks with the display electrodes 519 faced outside an original display area E.
The method of forming the alignment electrodes outside the display area of the display panel for providing alignment marks is superior in visibility as compared with the case of forming alignment marks with a material for forming a transparent electrode. However, because it is necessary to apply voltage to the electrodes, a circuit for voltage application has to be wired to the terminals of the electrodes, which decreases working efficiency in preparing the display panel.
In addition, in the method of forming the alignment marks in the area of a single electrode substrate where the electrode substrates of the display panel are not laid on each other beside the terminal forming area, and in the method of forming the shapes of the alignment marks in concentric circular shapes, the shapes of the alignment marks are different for individual display panels. On this account, a necessity occurs to change processes of forming alignment marks in preparing the individual display panels. Particularly, in a multi-layer liquid crystal display element in which a plurality of liquid crystal display panels for displaying different colors is placed in layers, the difference of the liquid crystal panels is only liquid crystals to be sealed between substrates, and the liquid crystal panels are allowed to have the same configuration and layout of the other members, such as electrode substrates, sealing wall materials, and end sealing materials. Thus, suppose that a process of forming alignment marks can be made common to all the display panels, fabrication efficiency will be improved dramatically.